Tuesday 18 December 2012 16.31 EST
First published on Tuesday 18 December 2012 16.31 EST

Downing Street turned its fire on the Metropolitan police when it called for an investigation into the "exceptionally serious" allegation that a serving police officer fabricated evidence against the government's former chief whip Andrew Mitchell.

Newly released CCTV footage appeared to raise questions about some aspects of the police account of the altercation between Mitchell and officers at the gates of Downing Street amid reports that one corroborating witness, the serving police officer, was not even present at the scene.

The official police log said "several members of public [were] present" during the incident. The footage shows that only one member of the public appeared to take an interest in the unfolding event.

Other pedestrians were seen walking past. The film has no sound, the police involved have had their identifies disguised and it is not possible to see Mitchell's face during the moments the comments are said to have been made.

Mitchell was forced to resign after the police log and an email by the unnamed officer suggested he had called policemen "fucking plebs" when they declined to let him to wheel his bike through the Downing Street gates. He admitted swearing at the police but has strenuously denied the toxic charge that he called them "plebs".

No 10 told police to "get to the bottom" of the matter. Craig Oliver, the Downing Street director of communications, issued a rare public statement after Channel 4 News broadcast the CCTV footage.

In an extraordinary twist to the story, which No 10 sources insisted was entirely coincidental, the police officer – the "witness" – emailed his account of the incident to his local MP John Randall, who was then Mitchell's deputy in the whips' office. Channel 4 said the police officer was also a constituent of Randall's.

Sources suggested that Randall, who had a strained relationship with Mitchell when he was appointed as his boss in September, had acted properly as an MP when he forwarded the email to No 10.

The officer told Randall: "Unfortunately I write to you to complain about the absolutely disgusting behaviour I was witness to yesterday of your fellow MP in Downing Street.

"I was with my nephew and … hoping to catch a glance of a famous politician. Other tourists standing with us were also shocked and some were even inadvertently filming the incident …

"I, having a keen interest in politics and the Conservative party, knew it was Andrew Mitchell, MP for Sutton Coldfield. Imagine to our horror when we heard Mr Mitchell shout very loudly at the police officers guarding [blanked out by Channel 4] our country. He just continued to shout obscenities at the poor police officers."

But the officer told Channel 4 News: "I wasn't a witness to anything."

Mitchell said the email, almost identical to the log compiled by police on duty at the time, was designed to kill his political career. Mitchell said: "It's very convincing unless you know it's untrue as I do and it was clearly aimed to destabilise me and finish me off by sending it into the heart of government to my deputy and could easily have done so very fast."

Mitchell demanded a full inquiry into the police account of events and insisted the email was key to the loss of his job. "I always knew that the emails were false, although extremely convincing," he told Channel 4. "It has shaken my lifelong support and confidence in the police. I believe now there should be a full inquiry so we can get to the bottom of this."

The Channel 4 revelations, after an investigation by its veteran political correspondent Michael Crick, cast the Mitchell affair in a new light and raise the prospect of a political revival for the former chief whip.

But Bernard Hogan-Howe, the Met's commissioner, said he stood by the officers who claimed Mitchell called them plebs. Speaking on LBC the commissioner said: "I don't really think from what I've heard up to now that it's really affected the original account of the officers at the scene … There's nothing I've seen in this fresh information that causes me to doubt that original account."

The Guardian understands that police have found nothing to suggest the log of the incident – which was leaked to national newspapers – is inaccurate or been tampered with.

No 10 indicated it has grave concerns about the police account. In a statement to Channel 4, Oliver said: "Any allegations that a serving police officer posed as a member of the public and fabricated evidence against a cabinet minister are exceptionally serious. It is therefore essential that the police get to the bottom of this as a matter of urgency. We welcome Bernard Hogan-Howe's commitment to achieve that aim." A criminal investigation is continuing into an officer who was arrested on Saturday on suspicion of leaking information about the altercation and misconduct in public office. He was arrested after the Met received new information on Thursday. They would not say where this had come from.

There were also questions for the Police Federation which called on Mitchell to resign after meeting him in his constituency. In a recording of the meeting, aired by Channel 4, Mitchell admitted swearing. He said: "The incident was very brief. I picked up my bicycle. I did say under my breath, but audibly in frustration: 'I thought you lot were supposed to fucking help us.' I did say that. It is for that I apologise."

The Metropolitan police federation strongly denied any conspiracy to get rid of a cabinet minister. John Tully, its chairman, said: "The Metropolitan police federation unequivocally and categorically refutes any allegation that it was part of a conspiracy to unseat a cabinet minister."

The Police Federation of England and Wales said in a statement: "We are aware that there is an ongoing investigation into this matter, we are therefore unable to comment further at this stage." Ken Mackaill, chairman of the West Mercia police federation, who attended the meeting with Mitchell, called on him to resign afterwards on the grounds that he had refused to outline what he had said.

Speaking after the meeting he said: "Mr Mitchell once again apologised for what he did say outside 10 Downing Street. He has also repeated a denial of key elements in a police report. For us the clear implication is that police officers have been dishonest. But he will not tell us what he did say. I think Mr Mitchell's position is untenable. I think he has to resign."

Fresh pressure on the Met came on Tuesday night from MPs on the powerful home affairs committee. The chairman of the committee, Labour MP Keith Vaz, said members were concerned about the allegations about police conduct and wanted answers. He said he would be writing to Hogan-Howe.

Vaz told the Guardian: "Following concerns expressed by members of the committee, I will be writing to the Metropolitan police for a full explanation of what happened."